We have investigated excision biopsy of anal skin tags as an adjunct to rectal biopsy in the diagnosis of Crohn's disease. Twenty-six patients with proven Crohn's disease of the large and/or small bowel were studied. All had perianal skin tags removed under local anaesthesia as outpatients, when rectal biopsies were also obtained. Three sections from each skin tag and three from each rectal biopsy were examined for granulomas. The rectal biopsies were also examined for changes ‘suggestive’ of Crohn's disease. Anal skin tags from 26 patients without Crohn's disease acted as controls. Of the patients with Crohn's disease, granulomas were found in both anal skin tags and rectal biopsies in five patients, in anal skin tags only in four, and in rectal biopsies only in three. When present, granulomas were more plentiful in anal skin tags than in rectal biopsies, being seen in all 3 sections in 7 of 9 ‘positive’ tags (i. e.: in 31% of 78 sections), compared to only 1 of 8 ‘positive’ rectal biopsies (i. e.: in 13% of 78 sections). No granulomas were seen in control anal skin tags. The procedure provides a simple technique which is complementary to rectal biopsy, by which histological confirmation of Crohn's disease may be obtained.